Measurement of the Spectral Shape of the $\ensuremath{\beta}$-Decay of $^{137}\mathrm{Xe}$ to the Ground State of $^{137}\mathrm{Cs}$ in EXO-200 and Comparison with Theory

Measurement of the Spectral Shape of the $β$ -Decay of ${}^{137}{Xe}$ to the Ground State of ${}^{137}{Cs}$ in EXO-200 and Comparison with Theory

S. Al Kharusi et al. (EXO-200 Collaboration)

Phys. Rev. Lett. 124, 232502 – Published 12 June 2020

Abstract

We report on a comparison between the theoretically predicted and experimentally measured spectra of the first-forbidden nonunique $β$ -decay transition ${}^{137}{Xe} (7 / 2^{-}) \to {}^{137}{Cs} (7 / 2^{+})$ . The experimental data were acquired by the EXO-200 experiment during a deployment of an AmBe neutron source. The ultralow background environment of EXO-200, together with dedicated source deployment and analysis procedures, allowed for collection of a pure sample of the decays, with an estimated signal to background ratio of more than 99 to 1 in the energy range from 1075 to 4175 keV. In addition to providing a rare and accurate measurement of the first-forbidden nonunique $β$ -decay shape, this work constitutes a novel test of the calculated electron spectral shapes in the context of the reactor antineutrino anomaly and spectral bump.